The 23-year-old is under the microscope anew after the World Anti-Doping Agency (WADA) said Saturday it was seeking a ban of up to two years after he twice tested positive for traces of the steroid clostebol in March.
The International Tennis Integrity Agency (ITIA) had cleared the Italian of wrongdoing in August, and weeks later he won the US Open for his second major title.
The ITIA accepted Sinner's explanation that the drug entered his system when his physiotherapist used a spray containing it to treat a cut, then provided massage and sports therapy to the player.
"First of all it's not a situation where I like to be in, it's a very delicate and difficult and also different situation," Sinner said in Beijing after defeating Jiri Lehecka in the quarter-finals 6-2, 7-6 (8/6).
"But what I know is that I always try to remind myself that I haven't done anything wrong.
"For sure I had sleepless nights during this time. Now again it's going to be not easy."
WADA has appealed the decision to clear him to the Court of Arbitration for Sport (CAS), reigniting the case.
"Obviously disappointed that I am again in this situation," said Sinner.
"I just try to stay somehow focused on my work and trying to do everything possible I can to be ready for each match I play.
"But yes, this is a very difficult moment for me and also for my team."
The ITIA found that Sinner "bears no fault or negligence" in the high-profile case and cleared him to carry on playing.
WADA on Saturday said: "It is WADA's view that the finding of 'no fault or negligence' was not correct under the applicable rules."
